Direct Diode Pumped  Ti:sapphire Lasers and Amplifiers

ABSTRACT

Direct diode-pumped Ti:sapphire laser amplifiers use fiber-coupled laser diodes as pump beam sources. The pump beam may be polarized or non-polarized. Light at wavelengths below 527 nm may be used in cryogenic configurations. Multiple diode outputs may be polarization or spectrally combined.

This invention was made with government support under grant numberDE-SC0009707 awarded by the Department of Energy. The government hascertain rights in the invention.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to using laser diodes to directly pumpTi:sapphire laser material for amplification of ultrafast pulses.

Discussion of Related Art

Many scientific and industrial applications can benefit from the use ofultrashort sub-100 fs duration pulses with moderate energy but at >100kHz repetition rates. Typically, Ti:sapphire oscillator/amplifiersystems have filled this role as a workhorse for research applications,with their unmatched ultrashort-pulse amplification performance andtunability. More recently, Yb-based fiber lasers and other ultrafastsources have become more-broadly adopted for less-demandingapplications, offering the advantages of a direct diode-pumpedarchitecture (i.e. light from a laser diode is used to energize the Ybmaterial that generates and amplifies the ultrashort pulses) at theexpense of pulse duration performance. In contrast, Ti:sapphire systemsrequire complex intracavity-doubled green lasers for the pump source.Nonlinear pulse compression or parametric amplification pumped byYb-based lasers can satisfy some needs for shorter pulses andtunability, but again this increases complexity and reduces thereliability of the source, and often generates pulses with largepedestal and other poor characteristics. Thus, the ideal ultrafastlaser—diode-pumped, simple, reliable, and with uncompromisingperformance—remains to be realized. This work demonstrates a Ti:sapphirelaser system that is scalable, robust, and high performance thatcombines the advantages of the Ti:sapphire gain medium with theadvantages of direct diode pumping that has previously only beendemonstrated in other less-ideal gain media.

Some useful background for the present invention is provided in thefollowing sources. Backus, S., et al., Direct diode pumped Ti:sapphireultrafast regenerative amplifier system. Optics Express, 2017. 25(4): p.3666-3674. Moulton, P. F., Spectroscopic and laser characteristics ofTi:Al2O3. Journal of the Optical Society of America B, 1986. 3(1): p.125-33. Durfee, C. G., et al., Direct diode-pumped Kerr-lens mode-lockedTi:sapphire laser. Optics Express, 2012. 20(13): p. 13677-13683.Moulton, P., Tunable Solid State Lasers. Proceedings of the IEEE, 1992.80(3): p. 348. Ellingson, R. J., et al., Highly efficient multipleexciton generation in colloidal PbSe and PbS quantum dots. Nano Letters,2005. 5(5): p. 865-871. Moulton, P. F., et al., Tm-Doped Fiber Lasers:Fundamentals and Power Scaling. IEEE Journal of Selected Topics inQuantum Electronics, 2009. 15(1): p. 85-92. Schuhmann, K., et al.,Thin-disk laser pump schemes for large number of passes and moderatepump source quality. Applied Optics, 2015. 54(32): p. 9400-9408.Honninger, C., et al., Diode-pumped thin-disk Yb:YAG regenerativeamplifier. Applied Physics B-Lasers And Optics, 1997. 65(3): p. 423-426.Yang, S. Y., et al., Multipass Ti: sapphire amplifier based on aparabolic mirror. Optics Communications, 2004. 234(1-6): p. 385-390.

SUMMARY

We successfully demonstrated the first direct diode-pumped ultrafastTi:sapphire laser amplifier. Furthermore, the current availability ofhigh-power 445-465 nm laser diodes and fiber-coupled modules isadvancing very rapidly, and now represents a reliable and cost-effectivesolution. Through novel design of the pumping geometry, we demonstratedfor the first time reliable pumping of Ti:sapphire with a non-polarizedpump laser source, and the first (to our knowledge) use of fiber-coupledpump light to implement a Ti:sapphire laser and amplifier. This isimportant in that a major advantage for continuous operation is thepositional stability of the pump light source in an ultrafast laseramplifier. The aggregate net result from this project is a verysubstantial advance in state of the art for ultrafast laser technology,with considerable promise for applications. The main invention is theuse of light directly from diode lasers, either unpolarized or polarizedand delivered either using discrete optics or optical fibers, for thepumping of Ti:sapphire lasers. We additionally disclose an enhancementin gain and overall conversion efficiency achieved through pumpingTi:sapphire amplifiers with light at wavelengths shorter than 527 nmwhile the laser crystal is maintained at low temperature, well belowambient and in some cases at cryogenic temperature.

A regenerative or multipass Ti:sapphire amplifier is pumped with directdiode light comprising single or multiple emitters. It may be cryocooled and/or pumped substantially below the peak absorption wavelength.It may include pulsed diode operation, and free space spatial and/orspectral combination of diodes as the pump source. It may include Fibercoupled combination of diodes as the pump source.

One example is a Ti:sapphire amplifier with two separately-pumped gainregions spatially separated in the amplifier crystal. An embodiment is aTi:sapphire amplifier in a thick disk geometry that is directly diodepumped and the pump light is recycled to pass multiple times through thesame spot on the crystal.

A specific embodiment comprises a system for amplification of ultrafastlaser pulses, including a seed source of light containing spectralcomponents within the Ti:sapphire gain bandwidth of 600-1080 nm, aTi:sapphire crystal as the gain medium, and a diode pump beam comprisingone or more diode lasers emitting within the Ti:sapphire absorptionbandwidth of ˜400-600 nm, where the diode pump beam is directed into theTi:sapphire crystal creating a population inversion, and the seed sourceis passed one or more times through the population inversion region ofthe Ti:sapphire crystal to effect gain.

In some cases the Ti:sapphire crystal is cooled to below 200 K. Thediode pump beam is comprised of the output from multiple diodes coupledinto a multimode fiber optic cable with a numerical aperture below 0.22.the diode pump beam may comprise the output from multiple diodes coupledinto a multimode fiber optic cable with a numerical aperture below 0.22and a fiber core diameter below 230 microns. Or, the diode pump beamcould comprise the output from multiple diodes coupled into a multimodefiber optic cable with a numerical aperture below 0.40 and a fiber corediameter below 105 microns.

The diode pump beam may consist of wavelengths between 435 and 480 nm.It may use polarization or spectral combining of multiple pump diodes,or both.

The seed source might be a mode-locked Ti:sapphire oscillator, possiblydirectly-diode pumped. A regenerative cavity can be placed around theTi:sapphire gain medium. Or, a multipass amplifier configuration couldbe placed around the Ti:sapphire gain medium. A combination of diodelaser sources and non-direct-diode laser sources can be used to pump theTi:sapphire crystal gain medium for enhanced gain.

In some embodiments the crystal is thinner than the confocal parameterof the pump light, and the pump light is re-imaged onto the crystal oneor more times to absorb a significant fraction of the pump light. Areflective geometry similar to the thin disk can be used, with one endof the crystal coated to reflect both the laser and the pump light, andwith an optical setup allowing for multiple passes of the pump.

The pump light might be low brightness from a fiber, or multiple pumpdiodes.

In a useful embodiment, the Ti:sapphire crystal gain medium is cooled tobelow 200 K and is pumped by a diode laser source with wavelength below480 nm, providing enhanced efficiency amplification of >1 excited statesper absorbed short-wavelength photon.

A Ti:sapphire amplification system has the pump light contained within abeam with high numerical aperture (NA>0.2) and is coupled into a crystalwith a path length between 0.5-1.5 absorption lengths, with crystaldoping at or below 0.25% Ti by weight, with a seed mode size 0.8-1.6times the pump spot size in the crystal, such that the amplification issubstantially optimized. The pump light is provided by one or more diodelasers coupled into a multimode optical fiber, for example with anumerical aperture below 0.22.

An amplification system embodiment for utilizing multiple pump lasersources contains a seed laser source, a laser crystal gain medium andone or more pump laser sources. The pump laser light from the one ormore sources is focused to more than one region in the laser crystalgain medium and the seed laser source is passed through the multiplepumped regions, enhancing the gain achievable from a single crystal. Auseful embodiment has the laser crystal gain medium as Ti:sapphirecooled below 200 K, and the seed laser source provides ultrafast laserlight. The seed laser source could be a modelocked Ti:sapphireoscillator, possibly directly-diode pumped.

The pump laser light could be provided by fiber-coupled diode lasers.One pump laser source is passed through the crystal and then reimagedinto a spatially separated spot within the same crystal. The seed lightis passed through the multiple pumped regions by a regenerativeamplifier cavity configured to overlap with all of the pumped regions,or by a multipass amplifier cavity configured to overlap with all of thepumped regions. The laser crystal gain medium might consist of two ormore separate gain crystals mounted in the same mechanical mount.

Another amplification system embodiment consists of a seed source oflaser light containing spectral components within the Ti:sapphire gainbandwidth of 600-1080 nm, a Ti:sapphire crystal gain medium that issubstantially less than one absorption length thick, mounted to a heatextracting element and providing reflection of pump and seed light onthe back surface, and a diode pump beam comprising one or more diodelasers emitting within the Ti:sapphire absorption bandwidth of 400-600nm. The diode pump beam is directed into the Ti:sapphire crystalmultiple times by pump beam imaging optics, creating a populationinversion and the seed source is passed one or more times through thepopulation inversion region of the thin Ti:sapphire crystal to effectgain. Again, the Ti:sapphire gain crystal may be cooled below 200 K

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram illustrating one configuration ofdirect-diode laser pumping of a Ti:sapphire laser or pulse amplifieraccording to the present invention.

FIGS. 2A, 2B and 2C illustrate different fiber coupled pumping schemesfor direct diode pumping. FIG. 2A illustrates pump sources coupledthrough a combination of lenses. FIG. 2B illustrates pump sourcesspectrally or spatially combined and then coupled through a combinationof lenses. FIG. 2C illustrates pump sources coupled through acombination of lenses into the different gain volumes.

FIG. 3A is a schematic detailed drawing for the pulsed amplifierconfiguration described in FIG. 1. FIG. 3B shows an alternateconfiguration where the regenerative amplifier portions of the cavityhave been replaced by an output coupler to generate a CW laser outputbeam.

FIGS. 4A-C illustrate a spatial combination scheme for three singleemitters. FIG. 4A shows single emitter laser diode elements collimatedand focused with lenses. FIG. 4B shows a fast axis view of the imageplane from the three emitter combination stage being relay imaged bylenses. FIG. 4C shows a slow axis view of the image plane from the threeemitter combination stage being relay imaged by lenses.

FIG. 5A is a plot showing the fluorescence yield from green blue lightas temperature is varied. FIG. 5B is an energy diagram for the Augerprocess that leads to an enhanced efficiency for below peak absorptionwavelength pumping of Ti:sapphire.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic block diagram illustrating one configuration ofdirect-diode laser pumping of a Ti:sapphire laser or pulse amplifier.Pump beam source 220 from a diode laser source 202 is coupled in to aTi:sapphire gain medium 205 using lenses 203, 204. A regenerativeamplifier cavity 207 is constructed around the Ti:sapphire gain mediumwith mirrors 208, 209, 210, 215, polarizers 211, 218, waveplates 213,217, 216, and a pockels cell 214, with the regenerative amplifier partsoutlined in box 222. An input ultrafast source 206 provides seed lightthat is switched into the cavity for a number of round trips by thepockels cell until it is switched out. A faraday rotator 212 causes theamplified light 223 to exit the output port of the input polarizer 211

FIGS. 2A, 2B, and 2C show different fiber coupled pumping schemes fordirect diode pumping. FIG. 2A: Pump sources 202 can be coupled through acombination of lenses 203, 204 into the same gain volume within theTi:sapphire crystal 205. FIG. 2B: Pump sources 202 can be spectrally orspatially combined and coupled through a combination of lenses 203, 204into the same gain volume within the Ti:sapphire crystal. The light canalso be retroreflected with a mirror to improve efficiency 221. FIG. 2C:Pump diode lasers 202 can be coupled through a combination of lenses203, 204 into the different gain volumes within the Ti:sapphire crystal,each creating a population inversion that can be extracted as pulseamplification by the amplifier cavity. FIG. 6 is a diagram of thedetails of a laser source 202 having a fiber interface. 201 is aspatially coupled laser diode module, 199 is a multimode fiber, and 200is a fiber collimator. Diode laser 202 has a fiber output of the diodelight, but lenses 203, 204 are still used to focus the fiber-outputteddiode light into the crystal 205.

FIG. 3A: Schematic detail for the pulsed amplifier configurationdescribed in FIG. 1.

FIG. 3B: Alternate configuration for block 222 where the regenerativeamplifier portions of the cavity have been replaced by an output coupler215 to generate a CW laser output beam 224.

FIGS. 4A-4C shows a spatial combination scheme for three singleemitters. FIG. 4A: Single emitter laser diode elements 230 arecollimated and focused with lenses 228 and 229. The three beams arespatially combined by mirrors 226 and overlapped to the image plane 227.FIG. 4B: Fast axis view of the image plane from the three emittercombination stage 225 being relay imaged by lenses 234-238 and generatea high brightness extended focal volume 239 within a Ti:sapphire gainmedium 205. FIG. 4C: Slow axis view of the image plane from the threeemitter combination stage 225 being relay imaged by lenses 234-238 andgenerate a high brightness extended focal volume 239 within aTi:sapphire gain medium 205.

FIG. 5A is a graph 240 of the fluorescence yield from 10 W of green 532nm light 242 and 10 W of blue 445 nm light 241 pumping the same modevolume of a Ti:sapphire crystal as temperature is varied from 20 degreesC. to −160 degrees C. The fluorescence yield for the green pumping 242follows the typical behavior described before, while the fluorescentyield for blue pumping 241 displays enhanced efficiency belowapproximately −90 degrees C. This points to the potential for anefficiency-enhancing Auger process. FIG. 5B is an energy diagram 243 forthe Auger process that leads to an enhanced efficiency for below peakabsorption wavelength pumping of Ti:sapphire. A high energy pump photon246 provides enough energy for long-wave fluorescence populated stateshigh in the ground state vibrational band 249 to can interact withelectrons injected high into the excited state band by the pump laser245 to create two excited-state electrons that can fluoresce 247, 248.

The successful demonstration of this invention required addressing asignificant number of challenges—it is not simply a case of swapping onepump source for another. The major parts to this invention are:

-   -   Combining high power single emitters into a single pump beam.    -   Maintaining pump laser brightness in a free-space or        fiber-coupled configuration.    -   Geometry issues when pumping with high NA beams combined into a        crystal kept in a cryostat.    -   Development of a low-loss cavity to function effectively with        the relatively low gain.    -   Thermal management.    -   CW pumping of a gain medium with relatively short ˜few μsec        lifetime.    -   Addressing the lower quantum efficiency of 450 nm compared with        532 nm.

We addressed all these major challenges in this invention.

Pump Beam Brightness

The absorption cross section for pumping of Ti:sapphire 205 peaks at˜490 nm. The absorption cross section is polarization dependent, and isconsiderably below its peak value at 450 nm. This increases thebrightness requirement for 450 nm pumping over that required for greenpumping since the doping density for Titanium in sapphire is limited toapproximately 0.25% whilst maintaining the figure of merit of thematerial (ratio of absorption at the pump wavelength to absorption atthe laser wavelength, which is ideally as high as possible). Thus, onemight expect that with the recent availability of ˜1 W single emitter520 nm diodes, this wavelength would be preferable for pumping. However,we found that the broad spectral bandwidth of the 520 nm diodes makeswavelength combination of the diodes difficult, and single emitters arenot available above 1 W average power. These two effects limit theability to combine multiple laser diode sources to increase theavailable pump power. Thus, the amplifier work to-date uses ˜440-465 nmdiodes 202. In this region, the intrinsic linewidth of individual lasersis narrow enough to use wavelength in addition to spatial combination tocombine many sources 230, each of which can have power of several watts.This enables the combined source 225 to achieve spatial brightnesssufficient for achieving gain in an amplification platform.

In our past work in direct diode-pumped Ti:sapphire modelocked lasers,the modelocking mechanism relied on a very tight focus of the pumplight—in that work, we demonstrated that a diffraction limited focus inone dimension of the pump spot was sufficient to implement stableKerr-lens modelocking. However, for a laser-amplifier system, theobjective is high average power. This will necessarily require a muchhigher pump power, originating from many single emitter diodes 202. Thiscan be made practical and repeatable by coupling the light into anoptical fiber. Although a fiber-coupled pump has many distinctadvantages (stability of pump mode size, shape, and position, as well aseasy pump laser replacement), it also presents manychallenges—scrambling of the pump polarization and reduced overallspatial brightness even as the total power is increased. For thisreason, other geometrical and spectral beam combination methods can beused to combine multiple emitters with only small reductions in spatialbrightness over a single emitter while maintaining polarization. Bothfiber coupled and free space pump sources can be used to directly diodepump Ti:sapphire.

Fortunately, many of these challenges have already been addressed in thedevelopment of high brightness fiber-coupled diodes in the infrared—workthat has been progressing for decades to a high level of refinement. Forthe initial demonstrations, 10 W total output from a 400 μm core, 0.22NA, did not provide sufficient brightness to demonstrate laser action.However, this exercise did provide significant insights into thechallenges involved. A second iteration was then successful: using 50 Wfrom a 200 μm core 0.22 NA fiber, the brightness from a single fiber wasincreased by 20×. This brightness was sufficient drive CW gain verycompetitive with that of current CW-pumped Ti:sapphire lasers, andsufficient to exceed the gain threshold even for a regenerativeamplifier with nearly 10% cavity loss. Future fiber-coupled laser diodesources expect to deliver to us a >75 W module, with up to a 2× increasein brightness over our currently-used sources. Several other companiesare rapidly increasing the average power and spatial brightness andfirst demonstrations of 150 W in a 200 μm, 0.22 NA fiber have beenaccomplished. Such sources are well-positioned to utilized the methodsdisclosed here for the direct diode pumping of Ti:sapphire.

In addition to fiber coupling, multiple single emitters 230 can bespatially and spectrally combined as shown in FIG. 4A. Three singleemitters 230 are spatially combined and imaged to an intermediate plane227 where the low M̂2 fast axis is used as the dimension for spatialcombination. The imaged and overlapped beams are essentially angularlymultiplexed in this geometry. Reimaging with differential magnificationin the fast and slow axis can then create an extended high brightnessfocal volume 239 within the Ti:sapphire gain medium 205 with a highpercentage of the absorbed light within a radius of 50 microns from thepropagation axis. This configuration, combined with single emittersof >3 W of power, gives achievable gains of 2× per pass; ideal for aregenerative amplifier configuration.

Cavity Geometry and Operation

One of the biggest challenges with direct diode pumping of Ti:sapphireis the delivery of the pump light to the amplifier crystal so that itgenerates a gain volume with sufficient population inversion and modeoverlap with the regenerative or multipass amplification beam path.Along with this issue is the mitigation of thermal issues from the highaverage power being delivered to the Ti:sapphire gain medium. With atotal pump power of 100 W from 2×50 W pump modules, it was clear fromthe beginning that any successful ultrafast laser-amplifier system wouldbenefit from the use of cryogenic cooling simply to manage thermallensing in the system—in general, Ti:sapphire lasers running at roomtemperature exhibit focal spot temperature gradients that create a verystrong thermal lens. In CW lasers, this is ameliorated by a very tight,focused laser mode in the Ti:sapphire crystal where the effect of thelarge thermal lens is ameliorated. However, the tight focusing approachwas not feasible with high power pumping and low spatial brightnesssources, so for some embodiments we implemented cryogenic cooling.

The very large NA of the pump light from the fiber-coupled diode 202required a very compact vacuum cryostat with windows as close aspractical to the crystal. To overcome this challenge, we employed a newcryostat 198 with a much smaller cold head. See FIG. 7. Laser gainmedium (e.g. Ti:sapphire crystal) 197 is disposed within cryostat 198,and both form the gain medium block 205. This allowed us to get the 100mm focal length, 50 mm diameter achromatic lenses close enough to thecrystal to 1:1 image the pump fiber tip onto the crystal. Although thiscryo is based on a different coolant than the recirculating heliumcryocoolers we typically employ, and operates at somewhat higher basetemperature (−160 C=113K vs 50K), it was sufficient to reduce thermallensing in the cavity where we could design a laser cavity to toleratethe expected thermal lens over a wide-enough operating range to make thecavity stable in practice. We additionally designed the regenerativeamplifier cavity to be tolerant to the thermal lens expected in highpump power operation.

In developing this cryocell, we quickly made a remarkable and unexpectedobservation. At room temperature and pumping with 50 W, we saw verylittle fluorescence compared to our experience with pumping at similarpower levels with 532 nm. (FIG. 5A) However, there is a dramaticincrease in the blue-pumped fluorescence when the crystal approaches−160 C. Work is ongoing to determine if this also corresponds to anincrease in gain and overall laser efficiency compared to 532 nmpumping, as the high thermal lens distortion precluded lasing at roomtemperature, and thus a direct comparison of low- and room-temperatureoperation. However, generally an increase in fluorescence willcorrespond to a high branching ratio of pump light to the lasertransition, or suppression of non-radiative decay pathways, both ofwhich also will generally improve laser operation and efficiency.

Florescence Yield Studies Based on 532 nm and 450 nm Pumping VsTemperature

To further investigate these large variations in fluorescence yield, wedid studies directly comparing fluorescence vs temperature when pumpedwith a polarized 532 nm source of the same deposited power. FIGS. 5A and5B show the resulting data. In both cases, we adjusted the powerabsorbed in the crystal to 10 W (200 um spot size in both cases), andused a spectrometer to observe the fluorescence intensityquantitatively. First—it is notable that although the 532 nm-pumped caseshows visible fluorescence at all temperatures (unlike 450 nm pumping),the fluorescence intensity nevertheless varies quite significantly withtemperature. This is consistent with previous measurements of theexcited state lifetime of the Ti:sapphire that showtemperature-dependent nonradiative decay competing with fluorescence.However, the character of the temperature dependence varies quitemarkedly for the two types of pump conditions. The 532 nm pumpfluorescence yield saturates in a manner expected as nonradiative decayprocesses simply become insignificant at low temperatures. However, at450 nm the yield continues to increase as temperature is decreased. Mostmarkedly, the yield for the 450 nm pumping exceeds the 532 nm pumpingfor temperatures below ˜−85 C.

The fluorescence spectrum in all cases shows no marked shifts, making itlikely that absorption in both cases populates the same excited-statemanifold. However, Ti:sapphire is known to be an efficient laser, with afluorescence yield of ˜1 at low temperatures. This means that theapparent fluorescence yield of Ti:sapphire may rise above 1 (to ˜1.5)when pumped with unpolarized blue light. These data are remarkable andrepresent a new possible mechanism advantageous for ultrashort-pulseamplification.

At first, the idea of an Auger-like excited state multiplication process(see FIG. 5B) seems unlikely—Ti:sapphire lases at wavelengths around 800nm, so that in a simple energy level scheme it would seem impossible fora photon at 450 nm to create 2 excited-state carriers. However, thebroad bandwidth of the Ti:sapphire gain spectrum would energeticallyallow this process—long-wave fluorescence populates states high in theground state vibrational band, which then can interact with electronsinjected high into the excited state band by the pump laser to createtwo excited-state electrons that can fluoresce. The intraband relaxationdynamics in Ti:sapphire are slow enough—gain quenching due to filling ofthe ground state is observed in Ti:sapphire for pulses shorter than ˜10ps duration. Thus, in CW pumping, this Auger mechanism—similar inmechanism to what has been observed for carrier multiplication inproposed next-generation photovoltaics—is possible. Similar processesalso occur for-example in the pumping of Tm:fiber lasers.

Diode-Pumped CW Ti:Sapphire Laser Demonstration

We tried several pumping schemes for maximizing absorption andvolumetric brightness of the pump light into the Ti:sapphire to maximizesingle-pass gain. Owing to the high NA of the pump delivery system,feedback of pump light into the delivery fiber is a major issue. Thistends to preclude, for example, simple pumping of the crystalsimultaneously from both sides. However, we were successful reflectingthe attenuated pump light back into the crystal for a second passwithout damage to the pump diodes. Using a cavity with 40 cm ROC opticsfor focusing the pump mode into the crystal, and using a cavity set-upfor maximum stability over a wide range of thermal lensing, wedemonstrated CW lasing.

For operation with 100 W pump, we were not successful in obtainingoverlap of pump spots from the individual pump modules in anyconfiguration, due to the large NA of the pump and feedback sensitivityof the diodes. So we designed a cavity with two foci in the lasercrystal to keep the two pump modules essentially independent. Thisconfiguration was successful in allowing us to test higher pump powersto verify our slope efficiency. Having two independent cavity focienables the utilization of counter-propagating pump sources withouthaving to overlap the pump sources in the same spot in the crystal,which can lead to optical feedback that can damage diode lasers. Thisconfiguration is a more compact than independently pumping two separategain crystals, and offers a simple way to multiply the amount ofavailable pump power. The utilization of this method at cryogenictemperatures helps to keep the thermal lens small enough that themultiple pump spots in the same crystal do not interact with each otherthermally, which is essential for cavity stability and efficientamplification. As higher spatial brightness sources become available, itwill also be possible to have additional pump spots and cavity spots incollimated space, reducing the complexity of the cavity design.

High Repetition Rate Pulse Amplification

By demonstrating laser action in a CW cavity (see FIG. 3B) with arelatively high-loss output coupler, we demonstrated the potentialfeasibility of implementing a pulse amplifier (see FIGS. 1, 3A). To thisend, we inserted polarizers 211, 218, wave plates 213, 216, 217, and anEO Pockels cell 214 into the cavity. This cavity can be optimized in aq-switched mode of operation, which we demonstrated with 1 W outputpower.

The next step is to seed the amplifier with an ultrashort pulse. Todemonstrate fully diode-pumped operation of a Ti:sapphireoscillator-amplifier system, in this work we used the modelockedTi:sapphire oscillator pumped by 520 nm laser diodes described later,although the amplifier could be seeded by any suitable ultrafast seedsource 206, such as a traditional oscillator pumped by adiode-pumped-solid-state (DPSS) green laser. This directly diode-pumpedTi:sapphire oscillator produces 40 mW at 68 MHz. The pulses pass througha polarizer 211, Faraday rotator (212)2/λ, and into the cavity throughan intracavity polarizer 218. The regenerative cavity 207 contains thepolarizer 218, a (e.g) halfwave plate 213, a quarter waveplate 217 and aKD*P pockels cell/driver 214. The pulses are trapped when the pockelscell 214 switches on, are held in the cavity for 162 round trips, andare then extracted back out through a Faraday rotator setup 212 toseparate the injected pulse from the ejected, amplified pulse 223.

To conclude, we have demonstrated for the first time, using acombination of novel approaches, that the direct diode-pumping ofTi:sapphire for pulsed regenerative amplifiers. The demonstration shownin this work is also applicable to multipass amplifiers. The spatial andspectrally combined source described in FIG. 4 will allow for enoughpump brightness to demonstrate a multipass amplifier in addition to aregenerative amplifier, which could enable pulses as short as 20 fsdirectly from the amplifier. Furthermore, we have discovered possiblenew physics in the Ti:sapphire system at low temperature when pumped byblue light, that may allow for especially high-efficiency operation.

Quasi CW Operation

To extend the utility of this invention beyond high repetition rateapplications, we can turn to techniques like current pulsing of thediode emitters for enhanced peak power. By pulsing the pump laserdiodes, it is possible to obtain higher brightness from the diodes atreduced duty cycle and thus repetition rate. To fully replace pulsefrequency-doubled Nd lasers would require a total cumulative energy inthe range of up to 50 mJ, delivered in a pulse duration comparable tothe ˜3 μsec excited state lifetime of Ti:sapphire. This corresponds to atransient power of ˜15 kW, which is likely not yet practical. However,for operation at tens to one hundred kHz repetition rate, even a 3-4×increase in quasi-CW power from the diodes at a 15-25% duty cycle wouldbe quite useful. For a practical system employing ˜500 W CW power of 450nm diodes (current cost ˜$250K), such a 2 kW quasi-CW power wouldcorrespond to ˜6 mJ stored energy in the Ti:sapphire, allowing for ˜1 mJpulse output @ 100 kHz (100 W). This would be a very useful laser forhigh-order harmonic generation as well as for applications such as thephotoinjectors for next-generation X-ray light sources.

It is well-known that pulsed operation of diode lasers can degrade theirlifetime, depending on whether peak- or average-power from the diode isthe lifetime-limiting factor. To our knowledge, no work has been doneto-date to evaluate quasi-CW operation of GaN and other UV/Vis diodes.Given that current single emitters at 450 nm have a rated lifetime of50,000 hrs (per Nichia), a reduction in lifetime of a factor of as muchas 10×, might be acceptable for research lasers, and possibly also forindustrial applications such as semiconductor metrology. Of course, inthis case, diode replacement must be cost-effective, with easyreplacement or redundancy. Once the prospects for quasi-CW operation aredetermined, the economics can be determined.

To investigate this issue, we did initial studies on quasi-CW operationof a single emitter 450 nm diode. Using a 1 W average power 450 nm diode(˜3 uJ energy over the storage lifetime of Ti:sapphire), we demonstratedan increase of 3.5× the average power switching at 40 kHz (5 us on time,10.5 μJ/pulse) without seeing any degradation over a short, 195 hr run.Thus, the initial evaluation of quasi-CW operation is promising.Implementation of quasi-CW pumping requires synchronization of thepumping with injection and extraction of seed pulses, and can reduce thecooling capacity required for the laser crystal.

Direct Diode Pumped Ti:Sapphire Oscillator Using 520 nm Diodes

Recently, industry has made advances in development of reliablehigh-power green laser diodes, with >1 W average power available in asingle-emitter diode. This work is motivated by lighting applications.Since the absorption cross-section for Ti:sapphire at 520 nm is high, weinvestigated their use for Ti:sapphire pumping. Although, as mentionedpreviously, the properties of the 520 emitters do not lend themselves tohigh average power pumping of Ti:sapphire amplifiers, they aresufficient to demonstrate direct diode pumping of Kerr lens modelockedTi:sapphire oscillators. We started by pumping a modelocked Ti:sapphireoscillator with 4×1 W single emitters, spectrally combined. These diodesexhibit relatively good beam quality—M²˜1.2 in the fast axis, and M²˜3in the slow axis, like the 450 nm diodes. The big difference is in thespectral width of the 520 nm diodes, which is 4× larger. This makesspectral combining challenging. However, considering the centralwavelength can be temperature tuned over a broader range, these diodesmay be feasible for use with wavelength combining, either exclusively inthis ˜520 nm band, or in combination with diodes in other bands such as405 nm, 450 nm, and 465 nm.

Alternate Approaches to Direct Diode-Pumped Ti:Sapphire Lasers

One of the major challenges for efficient operation of the directdiode-pumped Ti:sapphire lasers is in efficiently absorbing a verylarge-NA pump laser into a small lasing volume. Our success to-dateconclusively demonstrates feasibility, and the slope-efficiency of 26%is surprisingly good for a relatively low spatial brightness simple pumpbeam focusing geometry where we know, based on the geometry, that muchof the pump light is absorbed in regions of the crystal where the beamis poorly focused, and thus where this absorbed light does notcontribute to gain or lasing.

The challenge of pumping solid state laser with low brightness diodelasers is not unique to Ti:sapphire, and can for-example be effectivelyaddressed through the “thin-disk” pumping geometry. This approach, whichhas been used very successfully in other laser media such as Yb:YAG,places a relatively thin gain crystal at the focus of a parabola so thatthe pump beam can pass through the laser medium many times. In itssimplest manifestation, the crystal is high-reflective coated on theback and anti-reflective coated on the front for the pump and lasingwavelengths. For pumping Yb:YAG, a typical number of double-passes forthe diode pump is 16, and the optics can allow for as many as 50double-passes. The crystal is bonded on the back to a heat sink, so thatthe heat load can be extracted primarily in the direction of theamplified beam 223, greatly reducing the effect of thermal lensing.Therefore, if, for example, the absorption depth for the pump light is15 mm, the crystal thickness can be as thin as 150 μm and stillefficiently absorb the pump light. This short crystal thickness makes itmuch more feasible to use fiber-coupled laser diode light with extremelylarge M² values, requiring a tight focusing with a short confocalparameter for the pump light.

When applied to Ti:sapphire, the considerations are somewhat differentthan for Yb:YAG. In the case of Yb:YAG or Yb:Glass, this thin mediumallows for effective heat dissipation. However, sapphire is an excellentthermal conductor—better than copper at cryo temperatures. Thus previousinvestigations of thin-disk Ti:sapphire didn't observe a dramaticperformance improvement in a thin-disk geometry. Relative to Yb:YAG, thelongitudinal heat extraction aspect may not be as important, but the newfeasibility of direct diode pumping merits a re-evaluation of thethin-disk geometry for a Ti:sapphire amplifier. For Ti:sapphire, sincethe medium has limited doping concentration, the primary advantage is toabsorb the pump light in multiple passes in a laser crystal that isshort enough to keep the pump light near focus, but too short to obtainfull absorption of the pump in a single pass. The parabolic reflectorgeometry can allow, for example, for two diode pump modules to focus tothe same spot, and multiple pump passes can allow very efficient energydeposition into a single gain region. Since the “thin” aspect of theTi:sapphire crystal may be secondary, a “thick” disk pumping geometry,where the crystal is close to 1 mm thick, with parabolic pump focusingmay also be applicable.

What is claimed is:
 1. A system for amplification of ultrafast laser pulses, comprising: a seed source of seed light having spectral components within the Ti:sapphire gain bandwidth of 600-1080 nm; a gain medium comprising a Ti:sapphire crystal; and a diode pump beam source comprising a diode laser emitting within the Ti:sapphire absorption bandwidth of ˜400-600 nm; wherein the diode pump beam is directed into the Ti:sapphire crystal creating a population inversion; and wherein the seed source is passed through the population inversion region of the Ti:sapphire crystal to effect gain.
 2. The system of claim 1, further comprising a cooling device for cooling the Ti:sapphire crystal to below 200 K.
 3. The system of claim 2 wherein the pump beam wavelength is below 480 nm, and wherein the system results in amplification of >1 excited states per absorbed short-wavelength photon.
 4. The system of claim 1 wherein the diode pump beam source includes multiple diodes coupled into a multimode fiber optic cable with a numerical aperture below 0.22.
 5. The system of claim 4 wherein the fiber optic cable has a fiber core diameter below 230 microns.
 6. The system of claim 1 wherein the diode pump beam consists of wavelengths between 435 and 480 nm.
 7. The system of claim 1 wherein the diode pump beam source further comprises multiple pump diodes and a polarization combining device for polarization combining beams from the multiple pump diodes
 8. The system of claim 1 wherein the diode pump beam source further comprises multiple pump diodes and a spectral combining device for spectrally combining beams from the multiple pump diodes.
 9. The system of claim 1 wherein the seed source is a mode-locked Ti:sapphire oscillator.
 10. The system of claim 1, further comprising a multipass amplifier configuration placed around the Ti:sapphire gain medium.
 11. The system of claim 1 wherein the Ti:sapphire crystal is thinner than the confocal parameter of the pump beam, and further comprising optics configured to re-image the pump beam onto the crystal to absorb a significant fraction of the pump light.
 12. The system of claim 1 wherein the Ti:sapphire crystal is substantially less than one absorption length thick and includes a back surface configured to reflect the pump beam and the seed light.
 13. The system of claim 12, further including a heat extracting element attached to the Ti:sapphire crystal.
 14. A Ti:sapphire amplification system comprising a seed source of light; a gain medium comprising a Ti:sapphire crystal; and a diode pump beam source comprising a diode laser for coupling a pump beam into the Ti:sapphire crystal; wherein the pump beam is contained within a beam with a numerical aperture >0.2; wherein the crystal is configured to have a path length between 0.5-1.5 absorption lengths, crystal doping at or below 0.25% Ti by weight; and wherein the seed source is configured to provide a seed mode size 0.8-1.6 times the pump spot size in the Ti:sapphire crystal.
 15. The system of claim 14 wherein the diode pump beam source includes multiple diodes coupled into a multimode fiber optic cable
 16. The system of claim 15 wherein the fiber optic cable has a numerical aperture below 0.22.
 17. An amplification system comprising: a seed light source; a laser crystal gain medium; and a diode pump beam source; wherein the pump beam is focused to multiple regions in the laser crystal gain medium; and further comprising apparatus for passing the seed light through the multiple regions.
 18. The amplification system of claim 17 wherein the laser crystal gain medium is Ti:sapphire.
 19. The amplification system of claim 18, further comprising a cooling device for cooling the Ti:sapphire crystal to below 200 K.
 20. The amplification system of claim 17, wherein the seed light passing apparatus comprises a regenerative amplifier cavity.
 21. The amplification system of claim 17, wherein the seed light passing apparatus comprises a multipass amplifier cavity. 